diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5f3de2e89..b30e94b75 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -36,6 +36,9 @@
 
 ### v1.13.15
 
+**v1.13.15.9** *(2018-02-09)* - [[Userscript](https://raw.githubusercontent.com/ccd0/4chan-x/1.13.15.9/builds/4chan-X-noupdate.user.js)] [[Chrome extension](https://raw.githubusercontent.com/ccd0/4chan-x/1.13.15.9/builds/4chan-X-noupdate.crx)]
+- Fix 404 redirection on error pages without doctype. #1811
+
 **v1.13.15.8** *(2018-02-01)* - [[Userscript](https://raw.githubusercontent.com/ccd0/4chan-x/1.13.15.8/builds/4chan-X-noupdate.user.js)] [[Chrome extension](https://raw.githubusercontent.com/ccd0/4chan-x/1.13.15.8/builds/4chan-X-noupdate.crx)]
 - Captcha bypass cookie does not work for starting threads. Updating to treat this case correctly.
 
diff --git a/src/main/Main.coffee b/src/main/Main.coffee
index fee87ba84..5fee2a108 100644
--- a/src/main/Main.coffee
+++ b/src/main/Main.coffee
@@ -21,9 +21,6 @@ Main =
     try
       return if window.frameElement and window.frameElement.src in ['', 'about:blank']
 
-    # Don't run inside MathJax popups.
-    return if location.hostname is 'boards.4chan.org' and d.documentElement and not d.doctype
-
     # Detect multiple copies of 4chan X
     return if doc and $.hasClass(doc, 'fourchan-x')
     $.asap docSet, ->
diff --git a/src/site/SW.yotsuba.coffee b/src/site/SW.yotsuba.coffee
index 04fedab78..f2035638f 100644
--- a/src/site/SW.yotsuba.coffee
+++ b/src/site/SW.yotsuba.coffee
@@ -63,7 +63,7 @@ SW.yotsuba =
     # not 404 error page or similar.
     location.hostname is 'boards.4chan.org' and
     !$('link[href*="favicon-status.ico"]', d.head) and
-    d.title not in ['4chan - Temporarily Offline', '4chan - Error', '504 Gateway Time-out']
+    d.title not in ['4chan - Temporarily Offline', '4chan - Error', '504 Gateway Time-out', 'MathJax Equation Source']
 
   is404: ->
     # XXX Sometimes threads don't 404 but are left over as stubs containing one garbage reply post.